Porous rocks are saturated with fluids. The fluids may be water, gas or oil or a mixture of all three. The flow of current in the earth is determined by the resistivities of such rocks, which are affected by the saturating fluids. For instance, brine-saturated porous rocks are much less resistive than the same rocks filled with hydrocarbons. By measuring the resistivity of geological formations, hydrocarbons can be detected. Hence, resistivity measurements can be made in an exploration phase to detect hydrocarbons prior to drilling.
Various techniques for measuring the resistivity of geological formations are known, for example time domain electromagnetic techniques, as described in WO 03/023452, the contents of which are incorporated herein by reference. Conventionally, time domain electromagnetic investigations use a transmitter and one or more receivers. The transmitter may be an electric source, that is, a grounded bipole, or a magnetic source, that is, a current in a wire loop or multi-loop. The receivers may be grounded bipoles for measuring potential differences, or wire loops or multi-loops or magnetometers for measuring magnetic fields and/or the time derivatives of magnetic fields. The transmitted signal is often formed by a step change in current in either an electric or magnetic source, but any transient signal may be used, including, for example, a pseudo-random binary sequence. Measurements can be taken on land or in an underwater environment.
FIG. 1 shows a plan view of a typical setup for transient electromagnetic marine surveying. This has a bi-pole current source on or near the sea floor for transmitting a transient current between two electrodes. The time function of the current might be a simple step change in current or a more complicated signal such as a pseudo-random binary sequence. The response of the earth-water system is measured by a line of bi-pole receivers on or near the sea floor, each receiver measuring the potential difference between a pair of electrodes. All the electrodes are essentially in the same vertical plane. In use, the electromagnetic signal generated by the source of FIG. 1 can follow three transmission paths to the receiver electrodes, these being through the earth, through the water and through the air. In deep water, the airwave has a negligible impact. In contrast in shallow water, the signal that is transmitted through the water is negligible, but the airwave can have a significant impact and so make interpretation of the data difficult.